1. Field of the Invention
This invention relates to substrate processing apparatus and, more particularly, to an improved substrate lift finger for use in such apparatus.
2. Background
Substrates, such as semiconductor wafers, which require chemical processing are typically processed in thermal reactors. A large number of different reactors exist and, to illustrate the application of this invention, one such reactor (a chemical vapor deposition (CVD) thermal reactor) is illustrated in FIG. 1.
In this figure the reactor 10 is shown to include sidewalls 12, a top 14 and a bottom 16 which together define a thermal processing chamber 18 in which a substrate in the form of a semiconductor wafer 20 is located. The wafer 20 is supported on a susceptor 22 and can be lifted from the susceptor's surface by four lift fingers 24 which pass, from below, through holes 25 formed through the susceptor. The fingers 24 are mounted on a support hoop 26 and are moved vertically up and down under action of a drive motor 28 which is connected to the hoop at connecting tab 29. The susceptor 22 is able to move vertically up and down under action of a further drive motor 30 which is connected to the susceptor by means of an arm 31.
As a result of this configuration, the fingers are able to move upwards relative to the susceptor 22 to contact the underside of the wafer 20 and lift it from the upper surface of the susceptor 22. This operation is necessary so that the wafer 20 can be removed from the chamber 18. Once the wafer 20 has been lifted clear of the susceptor, a robot arm (not shown) enters the chamber through an aperture 32 formed in the back wall of the reactor 10. The robot arm is then positioned between the susceptor and the wafer 20 after which the fingers are moved downward until the wafer 20 is supported entirely by the robot arm. The fingers then move still further down to clear the underside of the wafer, after which the robot arm is retracted to remove the wafer 20 from the chamber 18.
Similarly, when a fresh, unprocessed wafer is to be inserted into the chamber 18 the robot arm brings the wafer into the chamber and holds it above the fingers 24. The fingers 24 are then moved up to engage the underside of the wafer to lift it from the robot arm. Thereafter, the robot arm is withdrawn from the chamber 18 and the fingers are moved down and the susceptor up until the susceptor 22 supports the wafer. After this wafer processing commences.
FIG. 1 also shows a bank of tungsten carbide heater lamps 34 which are located below the reactor 16. These heater lamps 34 serve to heat the susceptor 22 by means of thermal radiation which passes from the lamps through a quartz window 36 formed in the bottom wall 16 of the reactor 10. These heater lamps heat the susceptor which, in turn, heats the wafer 20 by means of conduction. During processing operations, processing gasses are injected into the chamber 18 by means of an injection fixture, commonly called a shower head 38, which operates to spread processing gasses evenly over the entire surface of the wafer.
The configuration of the conventional lift fingers 24 and their associated support hoop 26 is illustrated in greater detail in FIG. 2 which is an exploded pictorial view of the hoop 26. In this figure only one of the four fingers 24 is shown. The finger 24 (typically made of alumina ceramic) is generally U-shaped and includes an attachment tab 40 at the top of one leg and a wafer underside engaging tip 42 at the top of the other leg. The tab 40 of the finger 24 seats and is secured in a socket 44 formed in the hoop 26. The hoop 26 is also made of alumina ceramic and has an area that has been cut away so that it does not define a full circle. The cut away portion is formed to accommodate the arm 31 of the susceptor 22. This allows the hoop 26 to move close up against the underside of the susceptor 22 when the wafer 20 is lifted from the susceptor.
The lift finger 24 is secured to the support hoop 26 in a manner illustrated by reference to both FIGS. 2 and 3 in which FIG. 3 is a partially sectioned cross-section along the line 3--3 in FIG. 2 and shows the lift finger 24 secured in the seat 44.
A clamp 50 is bolted over the top of the tab 40 of the finger 24 by means of a clamp pin 52. The clamp pin 52 passes through both the clamp and the tab 40 and is secured at its lower end by means of a nut and a lock-nut combination 54 which bear against a Belleville washer combination 56. The Belleville washer combination 56 is made of two cup-like washers which are made of a resilient material and are placed back to back. The combination acts as a spring between the nut combination 54 and the tab 40. This ensures that the nut combination 54 does not bear directly against the tab 40 and thereby reduces the chances of fracturing the ceramic tab.
The clamp 50 is, in turn, fastened to the seat 44 by means of a pair of screws 58 which pass through the clamp and the portion of the hoop 26 below the seat 44 to screw into a plate 60 (not shown in FIG. 2) located underneath the hoop 26. As with clamp pin 52, the screws 58 bear against a Belleville washer combination 62. Furthermore, a pair of adjustment set screws 64 are provided to screw through the clamp 50 to bear against the seat 44.
During the assembly of the hoop, the fingers must be properly adjusted. This adjustment is required to (a) align the wafer supporting tips 42 of the fingers 24 with corresponding holes in the susceptor and (b) insure that all four wafer supporting tips 42 together define a flat, horizontal plane so that the wafer 20 can be properly supported by them. The adjustment of the lift finger 24 is achieved by adjusting the set screws 44 together with or independently of adjusting the pair of screws 58.
The above design has two major flaws. Firstly, apart from the lift finger 24 and the support hoop 26 (which are ceramic) and the Belleville washers (which are of a metal such as inconel) all the other components shown in FIGS. 2 and 3 are made from a commercial product sold under the trade name Haynes 240.RTM. by the Haynes International Company. Even though this material is very hard and very expensive to machine, it is necessary to use in this application as ceramic can not be formed with the screw threads required to provide the various clamping and adjustment functions. Apart from being very expensive to fabricate, Haynes 240.RTM. also has the disadvantage that, under the typical reaction temperatures in CVD chamber (about 470.degree. C.) it oxidizes and blackens over time. This oxidization/blackening could generate unwanted particles in the chamber and detrimentally affect the processing of the wafer.
The arrangement described above has a second disadvantage in that the fingers 24 are extremely difficult and time consuming to align correctly. This is primarily because a large number of different screws and bolts that need to be coordinated to produce the required alignment of the tips 42 of the four lift fingers 24. This problem is compounded by the plate 60 which is located beneath the ring 26 and which is very difficult to align with the screws 58. It has been found that the plate can be misaligned and not engaged correctly by the screws 58. This causes the plate to become dislodged during processing operations which, in turn, can cause damage to reflectors (not shown) located inside the chamber 18 as the hoop 26 is moved up and down under action of drive 28.
Accordingly a need exists for a different and improved configuration of wafer lift fingers and supporting hoop.